The invention is based on a measuring sensor for determining the oxygen content in gases. A measuring sensor is already known (DE-PS 23 38 536) which is provided with a protective pipe which encompasses, a segment of a measuring element (solid electrolyte pipe) essentially consisting of a ceramic material extending from a pipe-like metal housing. This protective pipe is provided with a plurality of openings at its side wall for the input of the measuring gases and a small opening in diameter, for the output of the measuring gases. The openings in the side wall of the protective pipe are so formed so that they essentially guide the measuring gases tangentially with respect to the inside face of the protective guide, and so that the measuring element is substantially protected against bombardment by the solid particles which are contained in the measuring gas.